This invention relates in general to strengthening wood-frame construction, and in particular, to a method of strengthening wood-frame construction and increase its resistance to high wind, earthquake or blast loadings by applying a reinforcement matrix comprising a resin and fibers to the panels.
A very common wood frame construction method uses wood or steel studs or wood or steel framing with plywood or Oriented Strand Board (OSB) sheathing panels or stucco sheathing. The framing/sheathing combination forms shear walls and horizontal diaphragms which resist horizontal and vertical loads applied to the structure. This form of construction is used in the majority of single family homes in North America, as well as a significant portion of multi-family, commercial and industrial facilities.
Wood composites comprised oriented strandboard (OSB) panels are increasing in popularity in traditional applications such as sheathing for roofs and walls, subfloors and floors. However, while OSB has become the dominant wood based sheathing material used in construction over the last 20 years, displacing plywood, the OSB has certain disadvantages, such as high vulnerability to thickness swelling and water absorption.
While the system has generally performed well, the economic losses in the United States due to natural disasters, such as hurricanes, earthquakes and tornadoes, have been mounting. The economic losses caused by these natural disasters in the United States have averaged about $1 billion/week in recent years. Most of these losses are due to hurricanes (80%) and earthquakes (10%). For example, loss of roof sheathing under hurricane winds has often been attributed to improper fastening of the sheathing to the framing, such as by the use of larger nail spacing than allowed by code, nails missing the support framing members, or over-driven nails. Loss of sheathing in hurricanes weakens the roof structure and can lead to roof failures. The water damage resulting from a loss of roof sheathing or roof failures has been a major contributor to economic losses in hurricanes. Surveys also show that a significant portion of the damage resulting from hurricanes or earthquakes occurs in nonstructural parts of the home due to excessive deformation or movements of the structure. The cost to repair nonstructural damage often makes it necessary to rebuild the structure rather than to repair it.
While the knowledge to mitigate hurricane and earthquake damage exists today, building code provisions are often misunderstood by builders, and compliance with regulations is difficult to enforce because of the difficulty of inspecting in the field. As a result, surveys show that a significant portion of the damage to homes and property caused by natural disasters is due to lack of conformance to codes. Improper connections between walls at building corners, such as non-overlapping top plates or improper or missing hold-downs to tie the shear walls to the foundations, are further examples of poor construction practices that are difficult to inspect.
Therefore, there is a need for a simple, easy-to-inspect, inexpensive construction method to strengthen and stiffen conventional construction for improved performance against hurricane and earthquake damage. The construction method should increase the strength and ductility of wood buildings and reduce the deformation of the buildings to limit damage to non-structural members.
In particular, many timber structures are situated in coastal areas that are continuously exposed to strong winds, salty and humid environments. Many factors in the environment, particularly water and temperature, as well as wind, earthquakes, insects, and fire affect timber structures. The most important factor leading to wood degradation and joint failures is, however, moisture. Moisture may penetrate the building envelope and then infiltrate into the fissures or micro-cracks existent in structural panels causing the system to deteriorate gradually.
It is, therefore, important that a building envelope provide a rain screen to prevent rain infiltration. It is desired that the building envelope be a continuous barrier in order to inhibit air leakage and to prevent the movement of moisture between the interior and exterior. It is important that the exterior building barrier is impermeable, or less penetrable, to the passage of moisture than the interior barrier. Moreover, the interior building barrier needs to provide a semi-permeable reinforcement, to allow the escape of moisture that has bypassed the inner barrier.
A common problem in the application of structural panels is durability of the connection zones subjected to load, mechanical wear and climate exposure. In particular, moisture uptake at the panel edges inflicts dimensional instability and deterioration of the material, which in turn causes connection failure.
Another problem that arises is the exposure of panels and connectors to moisture during the construction process. It is therefore desired to develop panels and connectors that will have improved dimensional stability and connector durability during the construction phase.
One potential method of protection against moisture penetration and increasing system durability of wood composites is application of coatings and/or reinforcements. In addition to moisture resistance, an effective edge protection system also offers reinforcement promoting dimensional stability and connector durability.
In the past coatings and/or reinforcements have been applied on the entire surface of a wood composite (i.e., covering the entire faces and edges), sealing the wood composite completely. However, perfectly sealed system is not easy to produce, but is expensive to manufacture, and is difficult to maintain. One disadvantage is that even a small discontinuity in such coating/sealing (a check or scratch through the protective layer) may allow moisture to accumulate inside the composite, and if such moisture is trapped inside the composite with no way out, over time the moisture destroys the composite.
U.S. Pat. No. 6,390,834 to Dagher and U.S. Pat. No. 6,699,575 to Dagher et al., which are owned by the same assignee as herein, describe applying fiber reinforced polymer strips to a wood sheathing panels used to build a structure or building to enhance the resistance of the structure to earthquakes and high winds from hurricanes and tornadoes.
It would be advantageous if there could be developed an improved system for improving the durability of a building system is by increasing the moisture resistance of its components (e.g., wood composites).